Superheterodyne receiver



Jail. 17, 193s).v A T, Wm; 2,144,355

SUPERHETERODYNE RECEIVER Filed Aug. 22, 1956- INVENTOR A A.T. WITTSATTORN EY I Patented Jan. '17, 1939 PATENT OFFICE 2,144,355SUPERHETERODYNETRECEIVER Alfred Thomas Witts, CheamySurrey, England,assignor to Radio Corporation of America, a corporation of DelawareApplication August 22, 1936," Serial No. 97,355

In Great Britain 7 1 Claim.

This invention relates to superheterodyne receivers and has foritsobject to provide improved means for overcoming a well knowndifficulty which frequently arises in known superhetero-" 5 "dynereceivers, namely that of image frequency reception.

It will be appreciated that when the local oscillator of asuperheterodyne receiver is tuned to a 'which will combine withthislocaloscillator frequency to produce the predetermined interme diatefrequency of the receiver, these twofrequencies being equidistant oneither side of the local oscillator frequency. Accordingly it may ocfcur that when a -superheterodyne receiver is tuned to receive from aparticular desired transmitter station there may be interference fromanother transmitting stationwhose frequency'is as much on one side ofthe local oscillator frequency as the frequency of the desired stationis on the other. Numerous attempts have been made to overcome this wellknown difficulty, i. e., to provide for image frequency suppression, butknown expedients for this purpose have not been entirely satisfactory.Known expedients for the purpose in question have been either undulycomplicated and expensive or not satisfactorily operative over a widerange of receivable frequencies or have been such as to involve loss ofain in the receiver or have presented combinations of thesedisadvantages. For example, it is well known to seek to overcome imagefrequency reception to some extent by utilizing a high intermediatefrequency, for example 450 kilocycles, but this expedient presents thedisadvantage of limiting the overall gain and furthermore results insome loss of adjacent channel selectivity as compared with that whichwould be obtained were a lower intermediate frequency chosen.

The present invention seeks to provide a simple and efiiective imagefrequency suppression means not involving the disadvantages abovementioned and satisfactorily operable over a wide receiving range, e.g., the present broadcast range.

According to this invention image frequency reduction or suppression ina tunable superheterodyne receiver is accomplished by means of afrequency selective rejector circuit or device which is arranged to bevaried as to its frequency when the tuning means of the receiver isvaried and is associated with a radio frequency circuit or a frequencychanger circuit of the receiver, said rejector circuit or device beingso arranged as to reject, at any given setting of the tuning means ofthe receiver, incoming signals whose frequency given frequency therewill be two frequencies September 21, 1935 (Cl. 25020); I

differs quantitatively from the local oscillator frequency bythe sameamount. a'sthat by Which the frequency desired to be received (atthesaid tuning setting) differs from the local oscillator frequency. I

Preferably the rejector circuit or device is a tuned circuit gangcontrolled with the high fre-' quency and local oscillator tunedcircuits of the quency circuit.

Though not limited to its application thereto the invention is ofparticular advantage in superhetercdyne receivers not involving highfrequency amplifier valves for it is in these receivers that, owing'tothe limited high frequency selectivity obtained, image frequency,interference is most serious. v

The invention is illustrated in the accompanying drawing which showsdiagrammatically one embodiment thereof. r

receiver and electronically coupled toa radio fre- V m Referring to thedrawing in which only those parts necessary to an understanding of thisinvention are shown a superheterodyne receiver has a so-called"triode-pentode valve I which is used both as a local oscillator and as afrequency changer; Incoming signals are applied to the innermost grid GIof the pentode portion of the valve, this grid circuit containing theusual radio frequency parallel tuned circuit Ll Cl, the condenser Clbeing, of course, a variable condenser for tuning purposes. Bias, forexample automatically Varied bias for automatic gain control purposes,is applied in any convenient manner well known per se to this innermostgrid. As shown automatic gain control bias is applied via lead A. V. C.and resistance Rl to grid GI, KI being a blocking condenser. The nextgrid G2 of the pentode portion of the valve is connected to a suitablesource (not shown) of positive potential through a parallel tunedcircuit L2 C2 in series with a resistance R2. The variable condenser C2is gang controlled with the variable condenser Cl as indicated by thebroken liine. The third or suppresser grid G3 of the pentode portion ofthe valve is connected to the cathode'C as in the usual way and theanode Al of the pentode portion is connected to the potential sourcethrough a parallel tuned circuit L3, C3, responsive to the predeterminedintermediate frequency of the receiver and in series with a resistanceR3. The cathode C is connected as shown to the potential source ends ofthe tuned circuits L2, C2 and L3, C3 through suitable condensers K2, K3.The triode portion of the triode-pentode valve is used as the localoscillator, the triode anode circuit containing a variably tuned circuitconsisting of a variable condenser C4 shunting a coil L4 in series witha fixed condenser C5. The condenser C4 is gang controlled with the othertwo variable condensers Cl, C2 and the coil L4 is magnetically coupledto a coil L5 which is connected in series with a resistance R4 in thecathode lead. The junction point of L5 wanna is connected to the triodeportion grid G4 through 1 suitable grid resistances R5 and the customaryshunt condensers K4 are provided as shown. The relative setting of thethree gang controlledcih cuits mentioned is such that in any givenposition the tuning of the local oscillator parallel tuned circuit L4,C4, C5, is midway between the tuning of the other two tuned circuits Ll,CI and L2, C2

se, e. g., by the utilization of small fixed con densersand/or by usingvariable condensers with specially shaped plates. Anode potential is-applied to the anode A2 of the triode portion of the gyalve throughresistance R6. 7 I

The invention is notlimitedto the particular embodiment above described.For example a triode-pentode frequency changer-local oscillator need notbe used, for obviously any type of frequency changer may be employed forexample 2. h'eptode or triode-hexode'and in cases where one or moreradio frequency amplifier valves precedethe frequency changer the imagesuppress- 40, mg variably tunable rejector circuit may be electronicallycoupled to any oneof these preceding high frequency valves.

Having now particularly described and ascer tained'the nature of saidinvention and in wha t manner the same is to be performed what isclaimed is: r

In a superheterodyne receiver, a combined detector oscillator circuitcomprising a tube having a detector section including a cathode, asignal grid, an auxiliary grid and an anode, and an oscillator sectionincluding said cathode, a grid electrode and an anode, a source ofuni-directional current having a positiveterminaland a negativeterminal, a grid circuit and an anode circuit for said oscillatorsection, said anode circuit including a.connection between the anode ofsaid oscillator section and said positive terminal, said grid circuitand anode circuitbeing coupled together for producing oscillations andincluding a. tunable'circuit for controlling the frequency oftheproduced oscillations, variable means-in said tunable circuit fortuning the circuit over a range of frequencies, an anode circuit for thedetector section of said tube comprising a connection between the anodeof the detector section andthe positive terminal of the source,

and a connection between the cathode of the tube and'the negative.terminal of the source, said anode connection including a circuitresonant to the intermediate frequency, means including, a tunablecircuit for connecting the auxiliary grid of the detector section to thepositiveterminal oi the source, said last named tunable circuit in.cluding variable means for tuning the circuit over a range of imagefrequencies, an input circuit for the detector section-of said tube;come prising a connection between the signal grid electrode and thecathode, said connection including a tunable circuit including, veriablemeans" for tuning the circuit overa range of signal ire-,-

quencies, and single means for varying said three variable meanssimultaneously and insuch a way that the oscillator frequencyismaintained half way between the frequencies to which the signalfrequency circuit and the image frequency circuit are tuned throughoutthe range of operation of the system.

" ALFRED THOMAS WITTSL

